1. Field of the Invention
The present invention relates to a correction method for a control apparatus controlling an injection molding machine, which method corrects a pressure detection value output from a pressure detector such as a load cell.
2. Description of the Relevant Art
As disclosed in Japanese Patent Publication (kokoku) No. 7(1995)-67720, in general, a control apparatus for controlling an injection molding machine includes a pressure control system which controls pressure such as injection pressure or back pressure, through feedback control, on the basis of a preset pressure command value and a pressure detection value output from a pressure detector such as a load cell. Such a control apparatus performs zero-point adjustment for the pressure detection value output from the pressure detector (load cell), in order to compensate for variation in the characteristics of the pressure detector itself and a mechanical error present at a portion where the pressure detector is attached. However, since the zero point set through an initial adjustment frequently drifts due to, for example, change in the external environment or long-term variation, in general, zero-point adjustment (re-adjustment) is performed regularly or as needed.
An example method for performing such zero-point adjustment is disclosed in Japanese Patent No. 2657352. In this method, a pressure detection value is obtained in a predetermined period during operation of an injection molding machine in which the pressure detection value theoretically becomes zero, such as a predetermined period after completion of initialization performed when power is activated or a predetermined period after completion of an injection step; and correction is performed on the basis of the thus-obtained pressure detection value.
In the above-described conventional method, since a pressure detection value is obtained in a period in which the pressure detection value theoretically becomes zero, optimal conditions are set in terms of period. However, in actuality, even in such a period, the pressure detection value varies depending on the state of the screw.
That is, when the screw is rotated in a state in which no pressure acts on the screw, as shown in FIG. 2, periodically-changing ripples, which are peculiar to each injection molding machine, are superimposed on a pressure detection value Pd output from a pressure detector, due to an error in a mechanism including a screw and a heating cylinder. Since the conventional method cannot avoid the error due to ripples, there is a limit in securing accuracy and stability of correction. In FIG. 2, a period from ts to ta and a period from ta to tb each represents one turn (revolution) of the screw.
An object of the present invention is to provide a correction method for a control apparatus controlling an injection molding machine, which method always secures a high degree of accuracy and stability in correction of a pressure detection value.
In order to achieve the object as described above, a correction method of the present invention comprises the steps of: rotating the screw by one turn in a period in which molding operation is not performed; obtaining a middle value between the minimum value and the maximum value of a pressure detection value output from the pressure detector during the rotation; rotating the screw further and stopping the screw when the pressure detection value output from the pressure detector becomes equal to the middle value; and correcting the pressure detection value output from the pressure detector after stoppage of the screw.